The viscous behaviour of strong and fragile glasses has been interpreted on the basis of the fragmentation model in which it is assumed that (1) a vitreous system consists of "pseudomolecules" and (2) the system is fragmented and the fragment size decreases with increasing temperature. The results of numerical calculations show that the size of fragments in viscous flow is constant for strong glasses whereas for fragile glasses it decreases as temperature is increased from T-g to T-m.